Heat pump water heaters are gaining broader acceptance as a more economic and ecologically-friendly alternative to electric water heaters. These systems utilize a condenser configured in a heat exchange relationship with the water storage tank, for example wrapped around the tank in a series of coils. During operation of the vapor compression heat pump cycle, a refrigerant exits an evaporator as a superheated vapor and/or high quality vapor mixture. Upon exiting the evaporator, the refrigerant enters a compressor where the pressure and temperature increase and the refrigerant becomes a superheated vapor. The superheated vapor from the compressor enters the condenser, wherein the superheated vapor transfers energy to the water within a storage tank and returns to a saturated liquid and/or high quality liquid vapor mixture. Conventional refrigerants are able to reject heat to the water in the storage tank via condensation in the condenser.
Carbon dioxide (CO2) has emerged as a natural, ecologically friendly replacement for CFC and HCFC refrigerants. CO2, however, has a low critical point and thus operates on a transcritical cycle wherein it evaporates in the subcritical region and rejects (transfers) heat at temperatures above the critical point in a gas cooler instead of a condenser. U.S. Pat. No. 7,210,303 describes a transcritical heat pump water heater system.
An impediment to wide scale acceptance of transcritical heat pump systems, including water heater systems, is the perceived lower efficiency of the transcritical CO2 vapor compression cycle as compared to the CFC and HCFC systems. In this regard, improvements are constantly being pursued to increase the thermodynamic efficiency and coefficient of performance (COP) of the transcritical systems. The present invention relates to such improvements.